The invention relates generally to the field of digital communications systems and more particularly to digital networks for facilitating communication of digital data in, for example, digital image, audio and video distribution systems and among digital computer systems. The invention is more particularly directed to a digital network in which message transfer paths may be provided with various classes of transfer service, including an unregulated available bit rate service and provides a mechanism for allowing the network to control the rate at which devices transmit messages over the network based on congestion along the path between source devices and destination devices.
Digital networks have been developed to facilitate the transfer of information, including data and programs, among digital computer systems and other digital devices. A variety of types of networks have been developed and implemented using diverse information transfer methodologies. In networks in which information is transferred using the well-known xe2x80x9cATMxe2x80x9d (xe2x80x9cAsynchronous Transfer Modexe2x80x9d) transfer methodology, communications are handled through a mesh of switching nodes. In an ATM network, the computer systems and other devices are connected to various ones of the switching nodes to, as information sources, provide information for transfer over the network and/or, as destinations, receive information from the network, with the information being transferred over paths, termed xe2x80x9cvirtual pathsxe2x80x9d and xe2x80x9cvirtual circuits,xe2x80x9d established through switching nodes comprising the network. The virtual circuits through an ATM network can be assigned to various types of service classes, which serve to govern the rate at which information, in the form of xe2x80x9ccells,xe2x80x9d is transferred thereover. In some service classes, virtual circuits are guaranteed maximum rates at which information can be transferred thereover. Other service classes, on the other hand, provide xe2x80x9cavailable bit ratexe2x80x9d (xe2x80x9cABRxe2x80x9d) services, for which virtual circuits are each guaranteed a minimum information transfer rate, referred to as a xe2x80x9cminimum cell ratexe2x80x9d (xe2x80x9cMCRxe2x80x9d) and can also make use of the bandwidth which has not been assigned to the virtual circuits which have the guaranteed maximum rates. For virtual circuits for which guaranteed maximum rates are provided, the sources already know the rates at which they can transmit information.
However, for virtual circuits which are associated with ABR service classes, the rate at which the sources can transmit may vary based on a number of factors, including, but not limited to, congestion conditions at the destination as well as congestion conditions at each switching node along the respective path from the source to the destination. To facilitate notifying each source of a virtual circuit which makes use of the ABR service class of the maximum rate at which it can transfer over the virtual circuit, the source periodically transmits a xe2x80x9cresource managementxe2x80x9d (xe2x80x9cRMxe2x80x9d) cell over the virtual circuit, the RM cell including the MCR assigned to the virtual circuit. When the destination receives the RM cell, it transmits the RM cell back to the source in the reverse direction along the virtual circuit. The destination can include an explicit rate value in the backward RM cell which indicates the maximum rate at which the source can tmsmit information. Each switching node along the path can also determine the maximum rate at which it can allow the source to transmit information and, if that rate is lower than the explicit rate contained in the backward RM cell, it can substitute its rate as the explicit rate value in the backward RM cell. Thus, when the source receives the backward RM cell, the explicit rate contained therein will be the minimum value generated by the destination and all of the switching nodes along the path defined by the virtual circuit.
The invention provides a new and improved system and method, referred to herein as a virtual bandwidth explicit rate marking system, for performing explicit rate marking of resource management (RM) cells in an ATM network.
In brief summary, the invention provides an explicit rate marking system for use in connection with a switching node to generate an explicit rate value for use in a resource management cell associated with a virtual circuit in an ATM network. The explicit rate marking system comprises a virtual bandwidth value generator module and an explicit rate value generator module. The virtual bandwidth value generator module generate a virtual bandwidth value reflecting an available bandwidth capacity value, an explicit rate value associated with each bottlenecked virtual circuit for which the switching node forms part of a path, and a minimum cell rate value associated with each unbottlenecked virtual circuit for which the switching node forms part of a path. The explicit rate value generator module generates the explicit rate value in relation to the virtual bandwidth value and the minimum cell rate value associated with the virtual circuit and other virtual circuits for which the switching node forms part of a path.